Projection displays are used for a wide variety of applications, such as producing the pictures viewed on television screens. A typical projection display system includes a number of components, including the screen, a light source, and an optical path between them. To create the pictures, one or more light sources are provided to emit light when it is needed. The light they produce is then manipulated by a series of optical devices in order to create the visual image. The visual image created along the optical path is then displayed on the visual-image display screen, the television screen for example, or another visual display. In most cases, the goal is to produce the best picture possible. The key to producing a desirable visual display, of course, is the configuration of the various optical devices along the optical path. The selection, operation, and configuration of these devices also contributes to unseen characteristics of the system, such as cost and efficient use of system resources.
Several types of projection displays have recently been developed. These new display systems are now becoming much more common, serving as a replacement for the widely-used CRT (cathode ray tube) display, which produces a visual image by producing and directing a stream of electrons at a treated display surface. The stream could only be directed to one point at any given time, but can be systematically swept across the display with such speed as to create the visual impression of a single image. This technology is fairly well-developed, but has reached the point where perceptible increases in quality are difficult to achieve. A CRT also takes up a relatively-large amount of space because the components used for generating the electron stream must be placed at a certain distance from the display screen. Many recently-developed projection display systems, in contrast, feature a much slimmer profile. In addition, projection display systems often produce much cleaner visual images. The combination of these advantages has made such systems immensely popular.
One such projection-display system is commercially available from Texas Instruments of Dallas, Texas under the trademark DLP® (or Digital Light Processing®). DLP® projection-display systems utilize a digital micromirror device (DMD) in their optical path. The DMD typically includes an array of thousands of tiny mirrors that are use to manipulate light originating at an internal light source. Other components in the optical path adjust the light for use by the DMD, or convey the image it generates. FIG. 1 is a simplified diagram illustrating an exemplary DMD-type projection display system 10.
In the display system 10 of FIG. 1, light source 11, which may be, for example, an arc lamp or an LED, emits light that first passes through a first condenser lens 12. Some light sources tend to produce a Lambertian emission and condenser lens 12 helps to produce a more focused (and more useful) beam of light, which then proceeds downstream to color wheel 13. (Note that the light-travel direction will for convenience sometimes be referred to simply as downstream). In FIG. 1, color wheel 13 has three sections; red, green, and blue, through which the light beam passes as the wheel rotates. Color wheel 13 may, for example, rotate once per frame of displayed image so that the light (when the light source is on) passes through each of the different sections in turn. Currently, a typical color wheel (not shown) may have as many as six to eight color segments and make two to three revolutions per frame.
After passing through the color wheel 13, the light passes through a second condenser lens 14 and then falls onto DMD chip 15. As mentioned above, DMD chip 15 includes thousands of micromirrors (as many as a million or more), mounted, for example, on a semiconductor chip. Note that for convenience herein, both the DMD and the chip-mounted device will be referred to simply as a DMD. Each micromirror is associated with a digital memory cell (not shown) and is mounted such that it can be individually adjusted so that light striking it can be selectively directed as necessary to create the visual image. This image is formed according to input from a source 16. Source 16 is shown as a single block that represents a variety of possible sources, for example a broadcast television station, a DVD, or a game playing device. While the source 16 provides indications of the image desired, control 17 generates the input for storage in the digital memory cells that will ultimately determine the position of each micromirror at any given instant. Light selectively reflected from DMD 15 for the image then passes from each of the selected mirrors then passes through a projection lens 18 so as to create the visual image on screen 19.
The visual image created on screen 19, of course, is a function of the position if each of the DMD micromirrors are selected at any given time. It is also a function of the quality of light that reaches the DMD 15. The use of color wheel has already been mentioned. In addition, the light may be intermittently blocked, altering the quality of light available for reflection by the DMD micromirrors. Current methods for blocking the light path, however, generally require the absorption of light by elements that form part of the optical path itself. Frequently, this produces problems related to the undesirable buildup of excess heat energy. Needed then, is a way to direct light in a projection display system so that is may be properly modulated while at the same time permitting discharge of unneeded light energy without excess heat buildup or the need for additional cooling measures. The present invention provides just such a solution.